* guix/scripts/package.scm (find-packages-by-description): Change 'rx'
parameter to 'regexps'.
[matches-all?, matches-one?]: New procedures.
Use them.
(process-query): Collect regexps from all 'search' queries, and pass
them to 'find-packages-by-description'.
* tests/guix-package.sh: Add tests.
* doc/guix.texi (Invoking guix package): Document it.